From an ancient soil sample, clues to the future of the ice sheet

In 1966, scientists at Camp Century, a now-abandoned U.S. military base in the Arctic, drilled deep into the Greenland ice sheet, unearthing a nearly mile-long cylinder of ice along with 12 feet of frozen sediment beneath.

“This was a pretty miraculous engineering feat that was really hard to replicate,” said Andrew Christ, a geologist who recently completed a postdoctoral fellowship at the University of Vermont.

The sample was the first deep ice core ever collected by scientists, and over the ensuing decades, the ice has become the subject of intense scientific study, providing crucial clues about the history of the planet’s climate. The same cannot be said of the deposits, which were largely ignored before they disappeared completely.

In 2017, sediment was discovered in a refrigerator in Denmark. Now, a study of frozen samples is shedding new light on Greenland’s past, perhaps offering an ominous warning for the future. the findings, published in Science On Thursday, he reports that nearly 400,000 years ago, the Camp Century site in northwestern Greenland was temporarily free of ice. They add to the accumulating evidence that the Greenland ice sheet has not been as stable over the past 2.5 million years as scientists once assumed.

“The big take home message from this is Greenland is at risk,” said Paul Berman, a University of Vermont geologist and author of the new study. “The ice sheet has melted in the past, so it could melt again.”

Dr. Berman W international team Several collaborators first began studying the sediment several years ago, and soon made a surprising discovery. The top layer of the sample, where they expected to find little more than a mixture of compacted rock, was filled with plant material: twigs, leaves, and small pieces of algae. discovery, Published by scholars in 2021indicates that the area was not always covered with ice.

“But the question we didn’t answer at the time was how old were these plants and sediments from this landscape that didn’t have ice?” said Dr. Christ, who is also the author of the new analysis. This new study in Science tells us when that happenedAnd which was 400,000 years ago.”

To get to that date, scientists used a technique known as fluorescence dating. When minerals sit in the ground, they are exposed to environmental radiation and free electrons accumulate. These electrons build up over time, but exposure to sunlight essentially removes electrons, because a washing machine might remove layers of dirt that build up on an item of clothing over the course of a weeks-long camping trip, Dr. Crist said.

By measuring the signal given off by the accumulating electrons, the researchers were able to calculate when the top layer of sediment was last exposed to the sun — and thus, how long ago the site was ice-free.

(Utah State University geoscientist Tammy Ritnor, who led this part of the study, had to analyze the samples in the dark to avoid “resetting” the electron clock.)

Once the scientists estimated the approximate date of the melt, they modeled different scenarios that could lead to an ice-free sampling site 400,000 years ago, calculating that the ice sheet should have melted enough to increase sea levels by at least four and a half feet.

“This is a significant rise in sea level,” said Dr. Crist. “And that’s something we really need to consider as a worst-case scenario of future climate change.”

He noted that the temperature at that time was not much higher than it is now, and carbon dioxide levels in the atmosphere were much lower.

There is still a lot of uncertainty about how the ice sheet will respond to continued warming, said Elizabeth Thomas, a University at Buffalo geologist and author of the new study. It’s difficult to extrapolate from that sampling location, she said, which is “close to the edge of the ice sheet and not also in a particularly sensitive part of the ice sheet.”

She said samples from parts of the ice sheet known to be less stable may be more telling about what might be happening as the planet warms.

“We have these amazing specimens that were collected in the ’60s,” said Dr. Thomas. “It’s great that we’re starting to work on it.” She added that it would be nice to “go back in time and say, ‘Hey, first ice drill team, could you please choose a different location?'” “